Vapour compression systems, such as refrigeration systems, air condition systems, heat pumps, etc., normally comprise a compressor, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant circuit. In the heat rejecting heat exchanger, heat exchange takes place between the refrigerant flowing through the heat rejecting heat exchanger and the ambient, e.g. in the form of a secondary fluid flow across the heat rejecting heat exchanger, in such a manner that heat is rejected from the refrigerant. Accordingly, the temperature of the refrigerant decreases as the refrigerant passes through the heat rejecting heat exchanger.
In the case that the secondary fluid flow across the heat rejecting heat exchanger is in the form of an air flow, the secondary fluid flow may be controlled by controlling a fan arranged in the vicinity of the heat rejecting heat exchanger. It is normally desirable to control the fan, and thereby the secondary fluid flow across the heat rejecting heat exchanger, in such a manner that the temperature of the refrigerant leaving the heat rejecting heat exchanger is close to the ambient temperature, e.g. close to the temperature of the fluid of the secondary fluid flow. Thereby it is ensured that the pressure of the refrigerant flowing through the heat rejecting heat exchanger is at an appropriate level, and that the vapour compression system is operated in an energy efficient manner.
In order to control the secondary fluid flow as described above, various temperatures, such as the temperature of refrigerant leaving the heat rejecting heat exchanger, the temperature of refrigerant entering the heat rejecting heat exchanger and/or various ambient temperatures, etc., may be measured. Each of the sensors used for measuring the relevant temperatures introduces an uncertainty of the measured temperature, which may result in a deviation between a measured temperature value and the actual temperature value. When the temperature of refrigerant leaving the heat rejecting heat exchanger approaches the ambient temperature, even small deviations in the measured temperature values may result in erroneous operation of the fan. For instance, if the temperature measurements performed by means of the temperature sensors indicate that the temperature difference between the temperature of refrigerant leaving the heat rejecting heat exchanger and the ambient temperature is above a desired level, but the temperature of the refrigerant leaving the heat rejecting heat exchanger is in reality as close to the ambient temperature as possible, the control algorithm may keep requesting an increase of the fan speed in order to further reduce the temperature of the refrigerant leaving the heat rejecting heat exchanger, but this will have no effect because it is not possible to reduce this temperature further. This is an undesirable situation, because it increases the energy consumption, the noise level, as well as the wear on the fan, without improving the overall energy efficiency of the vapour compression system.
WO 2013/156158 A1 discloses a method for controlling a fan of a vapour compression system, the fan being arranged to provide a secondary fluid flow across a heat rejecting heat exchanger of the vapour compression system. A temperature difference between a temperature of refrigerant leaving the heat rejecting heat exchanger and a temperature of ambient air is established. The temperature difference is compared to a first threshold value and a second threshold value, and the rotational speed of the fan is controlled on the basis of the comparison. The first and second threshold values are substantially constant.
US 2007/0125106 A1 discloses a supercritical refrigeration cycle comprising a heat rejecting heat exchanger in the form of a radiator, and a cooling fan for blowing the atmospheric air to the radiator. A value of information representing the difference between the actual radiation state of the refrigerant at the outlet of the radiator and an ideal radiation state determined by the atmospheric temperature is calculated, and based on this value of information, the air capacity of the cooling fan is controlled to decrease the difference.